1. Field of the Invention
The present invention relates to an electron beam exposure apparatus, and particularly to an electron beam exposure apparatus using a continuous stage moving method.
2. Description of the Prior Art
For the purpose of increasing the throughput of an electron beam exposure apparatus, a continuous stage moving method has been proposed.
The continuous stage moving method refers to a method of carrying out writing while moving a stage on which a wafer is held at a predetermined velocity. This method feeds back positional change of the stage to a deflector of an electron beam, and thus carries out an exposure as if a point of irradiation of the electron beam on the wafer did not move when viewed from an electron gun as a reference point.
Japanese Patent Application Laid-open Publication No. Hei 10-177941 has disclosed an electron beam exposure apparatus using a continuous stage moving method of this kind.
FIG. 1 is a diagram showing a schematic configuration of an electron beam exposure apparatus using a continuous stage moving method of a conventional type. The electron beam exposure apparatus includes an electron gun 13, a deflector 14, a wafer stage 15, a stage driving unit 16 and a feedback circuit 17. The electron gun 13 is to emit an electron beam 12. The deflector 14 is to deflect the electron beam 12. The wafer stage 15 is to fix a wafer 11 thereon. The stage driving unit 16 moves the wafer stage 15. The feedback circuit 17 feeds back positional changes of the wafer stage 15 to the deflector 14.
The feedback circuit 17 is configured of a laser interferometer 20, a controller 21, a corrector 22, a D/A converter 23 and an operational amplifier 24. The laser interferometer 20 reads the position of the wafer stage 15. The operational amplifier 24 is connected to the deflector 14. The controller 21 outputs a deflection signal for deflecting the electron beam 12 in response to the positional change of the wafer stage 15 which has been measured by the laser interferometer 20, and on a signal from the corrector 22. The D/A converter 23 converts this deflection signal from a digital signal to an analog signal, and the operational amplifier 24 amplifies the analog signal. Thus, the amplified analog signal is applied to the deflector 14. The feedback of the positional change of the wafer stage in this manner makes it possible to carry out the writing by causing the electron beam to follow a movement of the wafer stage no matter how the wafer stage moves.
As described above, the electron beam exposure apparatus using the continuous stage moving method of the conventional type carries out writing by causing the electron beam to follow the movement of the wafer stage no matter how the wafer stage moves.
A laser interferometer using a measurement frequency of approximately 10 MHz has been heretofore used for measuring a position of the wafer stage. For this reason, when the wafer stage moves at a velocity of 10 mm/s, a distance over which the wafer stage moves in a measurement cycle of 100 nanoseconds is 1 nm. As a result, an error in the stage feedback is as small as 1 nm. This does not cause a specific practical problem.
Recently, however, there has been an increasingly strong demand for carrying out the writing at a higher speed. It has become essential that the wafer stage be moved at a higher velocity. This has made it increasingly difficult to deflect the electron beam in order to follow the movement of the wafer stage.
FIG. 2A is a diagram showing how the position of a wafer stage changes when a frequency with which the position of the wafer stage is measured is 10 MHz and a velocity at which the wafer stage moves is 100 mm/s. FIG. 2B is a diagram showing how large a deflection voltage corresponding to the positional change of the wafer stage is, when a frequency with which the position of the wafer stage is measured is 10 MHz, and a velocity at which the wafer stage moves is 100 mm/s. When the velocity at which the wafer stage moves is 100 mm/s, the wafer stage moves over a distance of 10 nm in the measurement cycle of 100 nanoseconds. For this reason, even in a case where the wafer stage continuously moves, the electron beam is not deflected in the measurement cycle of 100 nanoseconds. This causes a 10-nm error in the stage feedback.
By contrast, if a high-precision measuring machine using a measurement frequency of not less than 10 MHz is employed, this employment makes it possible to hold the error in the stage feedback at less than 1 nm. In reality, however, it is extremely difficult to procure high-speed and high-precision machines of this kind.